I really like GDAX and Coinbase due to FDIC and USD pairs. I don't like the limited amount of coins they trade. Have you heard any rumors about them expanding the coins they trade?
They're definitely pushing into altcoins more and more. But they move much, much slower than other exchanges to add coins. BCH around the end of the year, hopefully more to come.
I don't know about GDAX but I am avoiding Coinbase because they have bigger fees than competition and the difference in price (buying vs. selling) is also too big on Coinbase.
GDAX has 0% maker fees. Don't use Coinbase. Transfer your funds to GDAX and trade for free. Market orders are expensive; don't use them.